Liquid ink printing may take a number of forms. In ink jet printing, exemplified by U.S. Pat. No. 4,544,931 (Watanabe et al), a liquid droplet is ejected from a single scanning nozzle and in U.S. Pat. No. 4,593,295 (Matsufuji et al) liquid droplets are ejected from multi-nozzle, multi-color heads arranged for scanning; in electro-osmotic ink recording, exemplified by U.S. Pat. No. 4,383,265 (Kohashi) ink droplets are made to fly from the tip of a needle shaped recording electrode; similarly, in electrostatic ink ejection, exemplified by U.S. Pat. No. 4,166,277 (Cielo et al), ink is retained in holes of an ink reservoir and is attracted out of the holes by the selective application of a voltage between the ink and selected electrodes; and in acoustic ink printing, exemplified by U.S. Pat. No. 4,308,547 (Lovelady et al), a liquid drop emitter focusses acoustic energy to eject a liquid ink. Our invention for sequencing the pattern of depositng ink droplets has equal applicability to each of these types of recording devices. It relates to the deposition of liquid ink onto selected pixel centers on command.
In a liquid ink recording apparatus, image quality is greatly affected by the physical properties of the recording substrate because the ink composition comprises more than 95% carrier liquid compared with only a small percentage of a suitable dye. The carrier liquid may be, for example, about 40% ethylene glycol and about 60% water. Since the desired marking material is only the dye portion, the remaining fluid must be driven off or absorbed into the recording substrate. This does not present a major problem with a paper recording substrate, because the paper has an affinity for the liquid. In fact, special coatings are usually applied to it for modifying and optimizing the diffusion isotropicity, diffusion speed, adsorption speed and reflection density of the deposited ink spots.
It is well known, however, that recording substrates of the overhead projection transparency film type present a problem in achieving high image quality because they have a poor ink spot diffusion capability. Although special coatings have been developed to shorten the ink drying time, the underlying Mylar.RTM. material is substantially liquid impervious and the drying time of liquid ink on these films does not approach the drying time on paper substrates. We have determined that it is the overlapping of still wet ink spots on adjacent pixel centers that causes a major image degradation problem referred to as "beading". When these adjacent ink spots impact the substrate and spread, ink from one spot will overlap into the region occupied by the other. This contact will disrupt the surface tension of the spots and ink will be drawn into the overlap zone depleting a portion of the ink from the remainder of the spot. As a result, the ink coverage will be non-uniform, causing a beaded, mottled appearance with alternate areas of high and low color saturation. This problem is aggravated when color mixing is required, since, in that case, each pixel area must be comprised of at least two superimposed droplets of ink and there is more ink to flow and bead between adjacent pixel areas.
In U.S. Pat. No. 4,617,580 (Miyakawa) there is taught a ink jet printing method for depositing drops of ink upon an overhead transparency film so as to obtain high color saturation. It is recognized therein that such a film does not absorb ink. In accordance with the Miyakawa invention, a plurality of smaller ink droplets are ejected onto a normal single-pixel area with the droplets being shifted slightly from one another by a predetermined distance. In FIGS. 3, 4 and 5, of the U.S. Pat. No. 4,617,580, there is shown a single pixel area upon which there are deposited, respectively, three, four and five smaller ink droplets.
In U.S. Pat. No. 4,575,730 (Logan et al) the non-uniform appearance of large area ink jet printing, referred to as "corduroy texture of washboard appearance" is attributed to non-uniform ink thickness "due to the thixotropic properties and surface tension". Better quality is attempted to be achieved by random overlapping of ink spots.
Although our invention will be described relative to a four-color multi-head scanning ink jet apparatus wherein each head is provided with plural nozzles, it should be understood that it is equally applicable to other liquid ink spot printing systems. In all these systems the spots are generally circular and high quality printing of graphic images, particularly solid areas, is achieved by overlapping adjacent spots so as to avoid uninked ("white") portions between ink spots. We have found that beading will inevitably occur on transparency film when overlapping spots of adjacent pixel areas are deposited while the liquid ink is still free to flow.
Therefore, it is the primary object of this invention to provide a method of depositing liquid ink spots upon an overhead projection transparency film, or the like, which will avoid ink flowing and beading between adjacent pixel areas, yet will result in output copy having good color saturation.
It is another object of this invention to provide a method of liquid ink spot deposition upon an overhead projection transparency film, or the like by which there will be no color banding between adjacent scanned lines of print when color mixing is effected.